Anatomy and physiology of the male reproductive system

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Introduction0:00–0:30

The male reproductive system includes internal and external organs and structures that help with reproduction. The external male sex organs are the penis, and below it, the scrotum.
Inside the scrotum, there are the two testicles, or testes - the male gonads. Inside the body, there’s a system of ducts through which sperm travel during ejaculation, as well as the male accessory sex glands, which secrete nourishing fluids for the travelling sperm.
Now, the testes are two organs the size of small plums, that are located in a skin and muscle pouch called the scrotum. This pouch has a line called the scrotal raphe running down the middle, which separates it in two chambers - one for each testis.

Scrotum0:30–1:11

The scrotum hangs outside of the body but has several layers of muscles and fascia that keep the testis temperature about 3 degrees lower than body temperature, which is perfect for sperm production.
When it’s cold outside, the scrotal skin wrinkles and the scrotum elevates to bring the testes closer to the body to warm up.
When it’s warm outside, the scrotal skin loosens up and the scrotum lowers the testes away from the body, and heat is released through sweating.

Testicles1:11–2:18

The testes themselves are covered on the outside by the tunica albuginea - a white, fibrous layer. If we slice a testis open and look inside - sorry for the cringe moment, guys - we can see that the tunica albuginea, sends fibrous projections called septa towards the center of the testis.
These septa partition each testis into about 250 lobules, and each lobule contains at least one and up to four seminiferous tubules, which is where sperm is synthesized.
The seminiferous tubules come together and form a single straight tubule that exits the lobule, and enters a small network of tubules called the rete testis.
The rete testis are a small network of ducts that split up and come together again - like catacombs, and these are located in the center of the testes - a region called the mediastinum testis.
Once the sperm make it through the rete testes, they go through the efferent ducts to the epididymis. The epididymis is a comma-shaped organ that also has a head, a body, and a tail, and curves along the posterior edge of each testis.
Now let’s zoom in on a single seminiferous tubule - which is the “sperm factory”. A seminiferous tubule has a thick wall of epithelial cells that surround a fluid-filled lumen - a bit like a garden hose.

Seminiferous tubule2:18–3:16

Outside the tubule, there’s connective tissue with capillaries, as well as Leydig cells - which produce testosterone. The wall of the tubule is made up of three kinds of cells: at the periphery, there’s the spermatogonia - which are the primordial sperm cells that begin dividing over and over in puberty, and give rise to male gametes.
Next, there are the spermatocytes - which migrate towards the lumen as they differentiate into sperm. Finally, there are Sertoli cells - which are large cells that extend from the margin all the way to the lumen of the tubule.
Sertoli cells provide nutrients to developing sperm cells, and contribute to the blood-testis barrier by only allowing certain molecules, like testosterone, in the seminiferous tubule.

Spermatogenesis3:16–6:13

Sperm production, or spermatogenesis, begins in puberty under the command of the hypothalamus. The hypothalamus is a part of the brain that secretes gonadotropin-releasing hormone, or GnRH.
That GnRH travels to the nearby pituitary, which secretes two hormones of its own - luteinizing hormone, or LH, and follicle stimulating hormone, or FSH, that reach the gonads.
Before puberty, GnRH and, in turn, FSH and LH are secreted in low, constant amounts. At puberty, secretion of these hormones intensifies and becomes pulsatile - sometimes more of the hormone is released, and sometimes less of the hormone is released.
LH binds to Leydig cells, and stimulates the production of testosterone, whereas FSH binds to Sertoli cells, making them produce androgen binding protein - or ABP, for short - which allows more testosterone to cross the blood-testis barrier and enter the seminiferous tubule.
So, it’s the increased concentrations of FSH, LH, testosterone that really gets spermatogenesis going. Spermatogenesis starts with the spermatogonia, which are diploid cells - so they have 46 chromosomes, two of which are sex chromosomes, an X and a Y.
For diploid spermatogonia to form haploid gametes, that only have 23 chromosomes, they first undergo mitosis, and then undergo meiosis 1 and 2.
Following mitosis, a spermatogonium gives rise to two 46-chromosome daughter cells: one of them eventually becomes a primary spermatocyte, and the other one becomes a spermatogonium - this way the population of spermatogonia stays constant.
The primary spermatocyte slowly moves towards the lumen of the seminiferous tubule, passing between two Sertoli cells that nourish it.
The primary spermatocyte enters meiosis, and emerges with 23 chromosomes each, one of which is either an X or a Y chromosome.
Secondary spermatocytes enter meiosis 2, which also have 23 chromosomes, but only one chromatid - so the right number of chromosomes and the proper amount of DNA.
Spermatids enter the lumen of the seminiferous tubule and undergo spermiogenesis - which is when they acquire a tail, and turn into sperm.
It takes roughly 2 months for spermatogonia to develop into sperm, and this process is regulated by various hormones. Sertoli cells secrete inhibin, that sends a negative feedback signal to the pituitary gland to reduce FSH production, and Leydig cells produce increasing levels of testosterone that send a negative feedback signal to the pituitary to reduce LH production.
All in all, this keeps the production of sperm at around 100 million per day - easier to remember as a whole lot of swimmers.

Sperm6:13–7:08

Sperm are made up of a few distinct parts. At the head is the acrosome, which contains enzymes that will help penetrate the oocyte - the female gamete.
Then there’s a middle piece - like a neck- which is loaded with mitochondria that provide the sperm with energy to move.
Finally, there’s the tail which helps the sperm swim. But when they’re inside the seminiferous tubules, the sperm haven’t learnt how to wiggle their tails just yet.
To learn this, they must exit the seminiferous tubules and go through the rest of the ducts inside the testis. The seminiferous tubules contract with a wave-like motion called peristalsis, and that moves the sperm through the straight tubules and into the rete testis, and eventually into the epididymis.
Sperm enters the epididymis through its head region, and are stored for a few months in the body region where they mature.
Here is where they begin to wag their tails and wait patiently for ejaculation. During ejaculation, the sperm that have matured exit through the tail of the epididymis, and enter the vas deferens.

Ejaculation7:08–8:44

The left and right vas deferens are two long tubes that are part of the spermatic cord on both sides and enter the pelvic cavity through the superficial inguinal ring.
The left and right vas deferens go behind the bladder and descend along the posterior bladder wall. Here, they dilate in a region called the ampulla, before narrowing again to continue as the ejaculatory ducts, which empty out into the first portion of the urethra.
The seminal vesicles - which are a pair of male accessory sex glands, lie adjacent to the left and right ampulla of the vas deferens, and they make seminal fluid which is loaded with fructose, an energy source for the sperm.
The seminal vesicles also empty out into the urethra at the ejaculatory ducts. Below the opening of the ejaculatory ducts, there’s the second male accessory sex gland, the prostate, a walnut-sized gland that wraps around the urethra, and makes prostatic fluid.
Beneath the prostate, on either side of the urethra, there’s a third pair of male accessory sex glands, called the bulbourethral glands.
The bulbourethral glands make a thick, clear mucus that acts as a lubricant. Together, the secretions of these glands nourish the sperm, and protect them from the acidic vaginal environment, giving them a better chance at reaching and fertilizing an oocyte.
All of these fluids and the sperm mix to form semen - which is ultimately ejaculated through the urethra. Now, the urethra is also the way urine exits the body, but during ejaculation, the bladder sphincter right above the prostate contracts, and that prevents urine from mixing with semen.

Penis8:44–10:53

Below the openings of the bulbourethral glands, the urethra is contained within the penis - so it’s called the penile urethra.
The penis ends with an enlarged tip, the glans penis, that has an opening called the external urethral orifice at its very tip, through which semen or urine are released outside the body.
Around the glans penis, a cuff of loose skin makes up the foreskin - that is sometimes removed during a medical procedure called circumcision.
On the inside, the penis is made up of three long cylindrical bodies of erectile tissue: the corpus spongiosum, which actually contains the urethra, and the two corpora cavernosa.
The corpus spongiosum also has an internal part, called the bulb, through which the urethra enters the penis, and the corpora cavernosa attach all the way to the pelvic bones.
Together, the internal parts of these erectile bodies make the up the root of the penis, which is embedded in the perineum.
The erectile tissues are made up of a maze of vascular spaces that give is a “spongy” appearance. These vascular spaces are lined with endothelial cells, and surrounded by smooth muscle cells, and the entire erectile tissue is wrapped in a fibrous tissue layer called the tunica albuginea.
During sexual arousal, the smooth muscle cells relax and more blood flows inside the vascular spaces. The corpora cavernosa distend, and they press against the inextensible tunica albuginea, compressing the veins in the penis, so blood is unable to drain away.
The corpus spongiosum has a more flexible tunica albuginea, and so becomes less turgid, avoiding compression of the penile urethra.
With blood coming in, but no blood leaving - there’s local engorgement, which is called an erection. Sometimes this can happen during sleep even without sexual arousal - a phenomenon called “nocturnal penile tumescence”.
All right, as a quick recap: the male reproductive system comprises of internal and external sex organs that help with the production, transport and release of sperm - the male gametes.

Review10:53–11:30

Sperm production begins at puberty, and continues throughout the entire lifetime under the control of hypothalamic and pituitary hormones.
Sperm goes to the epididymis for maturation, and it’s released from the epididymis during ejaculation - traveling through a long and winding road of ducts along the way - the vas deferens, the ejaculatory ducts, and finally, the urethra, which opens at the tip of the penis.